The present invention relates to a method for oxidising (and optionally removing) manganese and other inorganic species in aqueous solutions. For example, the invention relates to the treatment of manganese and other inorganic species in potable water, industrial waste waters and process liquors.
Many drinking water supplies across the world are contaminated by trace contaminants such as manganese, arsenic and heavy metals. Manganese is also present in mining and mineral processing effluents. The removal of trace contaminants to very low concentrations is often required for aesthetic reasons (e.g. the presence of manganese gives rise to xe2x80x9cdirty or black waterxe2x80x9d problems and can result in soiling of clothes and staining of household fixtures when present in concentrations in excess of 0.020 mg/L in drinking water) or for health reasons (the WHO drinking water guideline for arsenic is 0.010 mg/L).
Manganese removal is often difficult because dissolved divalent manganese (Mn(II)) is poorly adsorbed by coagulants commonly used for water treatment such as iron and aluminium compounds. Consequently, the removal of trace manganese requires a pre-oxidation step in which dissolved manganese(II) is converted to the insoluble manganese(III) and/or (IV) oxides (or oxyhydroxides). Thereafter, a coagulation step using iron or aluminium salt can be used to remove the manganese oxide particles.
The oxidative precipitation of manganese in ambient conditions, however, requires powerful oxidants such as permanganate (the oxidation rate using chlorine is usually too slow) which can be expensive and difficult to handle.
The removal of arsenic from process liquors by the oxidation of iron(II), arsenic(III) and sulfur(IV) with oxygen has been studied (T. Nishimura et al., xe2x80x9cRemoval of Arsenic from Process Liquors by Oxidation of Iron(II), Arsenic(III) and Sulfur(IV) with Oxygenxe2x80x9d, Proceedings of the second International Symposium on Iron Control in Hydrometallurgy, CIM, Montreal, Ottawa, Canada, 1996, 535-547). However, it was disclosed in this paper that in the presence of dissolved iron, the oxidation reactions (As(III) to As(V), and Fe(II) to Fe(III)) were only effective at 2 less than pH less than 5.
Surprisingly, the present inventors have discovered that the addition of iron(III) compounds in neutral or alkaline aqueous solutions can accelerate the rate of oxidation of manganese and other inorganic species in the presence of oxygen and sulfur(IV). This is despite the fact that iron exists in a solid (precipitated) form in these solutions.
Accordingly, the present invention provides a method for oxidising an inorganic species in an aqueous solution of pH 5 or greater, comprising the steps of:
(i) supplying an oxidisable form of a sulfur compound, and oxygen to the solution; and
(ii) adding a source of iron to the solution and allowing oxidation to take place wherein said source of iron provides an iron based catalyst for the oxidation reaction.
Typically the source of iron is a soluble iron(III) compound such as ferric-chloride or sulfate.
Alternatively, iron(II) can be supplied to the solution in a form which can be readily oxidised to iron(III) (eg. as ferrous sulfate), which then accelerates the oxidation reaction.
Typically the species oxidised is manganese and preferably oxidation is effected by the addition of sodium sulfite and oxygen (air) in the presence of an iron compound (eg. as precipitated iron compounds). Furthermore, the product resulting from the added iron can subsequently (and advantageously) serve as a coagulant to remove the oxidised species from the solution.
Oxygen is advantageously used as the oxidising agent because it has no residual contaminating after-effects. Sulfur sources can be selected, (e.g. sulfite or SO2 gas) such that in the oxidising procedure, a relatively benign product is produced (eg sulfate). Whilst the final product of using sulfite is a relatively benign dissolved sulfate, it is still preferable to use it sparingly, especially if an ion-exchange process is subsequently used to remove the contaminant (e.g. arsenic). In this latter case residual dissolved sulfate of no more than 25 mg/L is preferred, as this then enables effective arsenic(V) removal (i.e. sulfate and arsenate may otherwise compete for sites on the ion-exchange material).
The oxidisable sources of sulfur can be SO32xe2x88x92, S2O32xe2x88x92, S4O62xe2x88x92, SO2(g), aqueous SO2, or HSO3xe2x88x92. However, sulfur dioxide and sulfite are the most preferred sources. Also, waste sulfur dioxide gas may be available for use in industrial applications.
Typically the process is applied in the treatment of trace quantities of inorganic species but the process can also find application with more concentrated quantities of contaminants (e.g. in geothermal waters, leachates obtained from smelter wastes, industrial process liquors etc).
In addition to manganese, the species oxidised can include one or more of arsenic, sulfide, selenium and may also include uranium, cobalt, antimony, bismuth and other inorganic species.
Typically the oxygen is sparged into the aqueous solution as air but other methods of addition are possible As indicated above, the solution is typically a drinking water solution, an industrial waste water or process liquor etc.
Typically the pH of the solution is, if necessary, made to be near neutral or basic.